What is the difference between the contraceptive patch and the plan b pill?

The Ortho-Patch is birth control, Plan B is Emergency Contraception.

The patch is used before, during and after sex - it is designed to be worn 24 hours a day, 21 days per month. It continuously delivers a low dose of female hormones that mimic pregnancy to prevent ovulation and thicken mucous to stop sperm from entering the uterus through the cervix.

Plan B and all ECPs used after unprotected sexual contact, act in much the same way as the patch, NuvaRing and oral contraceptives - by immediately sending a high dose of hormones that tries to stop sperm already in the uterus, from reaching the destination (a fertile egg) or postponing ovulations until several days have gone by and no active sperm are still in the reproductive tract.

The differences are; the low dose of BC hormones will regulate your system while the high dose will most likely throw off your cycle for a few weeks. The low dose has few side effects in most women, while the high dose may cause nausea.

The big difference is by using Birth Control 24/7, without any other BC (like a condom) you have a 1/200 chance of getting pregnant in a year. By adding a condom the chance is even less.

By using Plan B within 72 hours of unprotected sex, 2 to 12 out of 100 women will become pregnant in a year.

Neither pills, nor patch protect you from STI's.